Earth’s climate is approaching irreversible tipping points

The Amazon rainforest is so big that it makes its own climate. As they photosynthesise and transpire, its billions of trees collectively produce enough moisture to form clouds. These, by some estimates, are responsible for at least a third of the rainforest’s life-sustaining rainfall. But climate change is disrupting this circular process. The build-up of greenhouse gases in the atmosphere has raised regional temperatures, worsened droughts and increased the risk of fires. All kill trees.

Fewer trees means less rainfall, higher temperatures and yet more fires. Climate-change-induced deforestation therefore risks becoming self-perpetuating. And the more humans with chainsaws do to help things along, the sooner the dire day will come when the forest has shrunk so far that nothing can be done to restore it. Much of the basin will turn into a dry savannah, and the tens of billions of tonnes of carbon dioxide stored there will be released into the atmosphere, further heating the planet.

“Amazon dieback”, as this grim scenario is known, is just one example of what climate scientists refer to as a tipping point: a threshold beyond which self-sustaining processes irreversibly push a part of Earth’s climate system from one state into another. Those who study them think there are many other examples. These include the breakdown of the vast Greenland ice sheet, which would raise global sea levels by more than seven metres, and the collapse of the Atlantic Meridional Overturning Circulation (AMOC), the powerful system of heat-distributing ocean currents that keeps northern Europe reasonably temperate. Should AMOC collapse, temperatures and rainfall levels could fall dramatically across Europe, greatly damaging the continent’s ability to grow crops.

In the 20-odd years since this way of thinking about the climate became formalised, the scientists involved reckon they have arrived at a decent—though not perfect—understanding of which parts of the climate system are most vulnerable to tipping, and why. Now they, along with politicians and business leaders, are trying to answer other, increasingly pressing questions: how to tell if a tipping point is actually being crossed, for one, and how to prepare for the consequences if it is.

The exact level of warming required to trigger any specific tipping point is not clear. Earth’s climate is governed by myriad interconnected processes, many of which—like the dynamics governing ice-sheet disintegration, or the potentially cooling effects of wildfires—are only poorly understood. Others, such as the formation of light-reflecting clouds, occur at scales too small to be properly incorporated into planetary models. To further complicate things, one tipping point can trigger another, domino-style. The fresh water released into the oceans from a collapsing Greenland ice sheet, for example, would weaken AMOC, further reducing rainfall over the Amazon.

Different models, therefore, rely on different approximations and make different projections of when tipping points will occur. Some models suggest, for instance, that the Greenland ice sheet could start to enter an irreversible decline once global temperatures are 0.8°C above pre-indus­trial levels—something that happened around the turn of the millennium. Others put the threshold at closer to 3°C—which might never be reached. Similarly, the Amazon’s decline is projected to become unstoppable somewhere between 2°C and 6°C of warming, though it could be greatly hastened if humans keep cutting down or burning trees at current rates.

It may thus be possible to defer the Amazon’s tipping point simply by reducing deforestation as much as possible. Averting others, though, depends on the bigger and more difficult task of limiting how much global temperatures rise. And, with the global average now 1.2°C above pre-industrial levels, and projected to breach 2°C by the end of the century, it is unclear how much time is left in which to do so. That makes it ever more important to get a sense of whether any of these tipping points are already being crossed.

In order to help answer that question, Britain’s Advanced Research and Invention Agency (ARIA) announced in February that it was going to fund systems that could produce and process the data needed for an “early warning system for tipping points”. ARIA’s initial five-year, £81m ($109m) programme involves 26 teams focusing on two tipping points in particular: the breakdown of the Greenland ice sheet and the collapse of the subpolar gyre, a circulating current in the north Atlantic which helps power AMOC. If too much fresh water from melting ice flows into the gyre, it could be disrupted, increasing the odds of an AMOC collapse.

Kelly Hogan, a marine geophysicist at the British Antarctic Survey (which, despite its name, is functionally bipolar), is co-leading one of the teams focused on the Greenland ice sheet. They plan to use a fleet of small underwater drones to both map the shape of the ice face and measure properties such as salinity, temperature and the force of currents. These data will shed light on the way temperature and salinity change at the interface between ice and water—things scientists expect to influence melting. They will also deploy robots that can roam the surface of the ice taking measurements and drilled-in sensors for longer-term monitoring.

Other teams are following a similar logic. Oshen, a British startup, intends to deploy small, self-sailing robots with solar-powered sensors in the subpolar gyre, where they will measure such things as sea and air temperature and wind speed. Marble, another British company, is developing drones that can monitor the position and size of icebergs, the location of the glacier front and the height of the Greenland ice sheet, three variables that are essential to accurately forecast melting.

Both Oshen and Marble say their work is only possible because smartphone technology has made sensors and processing power cheap. Control systems that once required proprietary software can now be run using free, open-source code. And widespread 4G coverage means that data can be transferred quickly. “We’re not inventing some new breakthrough laws in physics,” says Mathieu Johnsson, Marble’s CEO. “We’re exploiting a lot of technologies that have been there for a little while…it’s just that they haven’t been put together in the right way.” Meanwhile, several other ARIA-funded teams—including one led by Tim Lenton, a climate scientist at the University of Exeter and a pioneer of tipping-point research—are working out how these data might inform an early-warning system.

For all this to be useful, says Dr Lenton, policymakers need to think more about the consequences of tipping points being crossed, and how societies must prepare for them. Laurie Laybourn, who leads the Strategic Climate Risks Initiative, a British think-tank, agrees. “The mental model of the climate threat among key people—particularly in senior parts of government—has yet to catch up with the fact that the nature of the climate threat includes things like tipping points,” he says. In his view, no government is considering scenarios like ice-sheet collapse with the seriousness afforded to other high-impact risks, such as pandemics. In fact, Mr Laybourn reckons, with the possible exception of the Nordic countries, most governments have not really been thinking about them at all.

For some, talk of tipping points is a harmful distraction. In 2024 an international group of well-known scientists published an article in Nature Climate Change arguing that a focus on tipping points diverted attention from the more general need for climate mitigation and adaptation, around which the science is much more certain. Others worry about fostering a sense of fatalism, by framing some catastrophic changes as unavoidable.

Regardless, the concept is slowly but steadily gaining ground. In July a big conference on tipping points in Exeter attracted actuaries, insurers and pension funds as well as scientists and activists. Emergency services and humanitarian organisations are showing increasing interest, too. And so are the Brazilian organisers of COP30, this year’s United Nations climate summit, who are expected to place particular emphasis on the subject. The conference is being held in November in Belém, a city dubbed “the gateway to the Amazon”. The setting could scarcely be more apt. ■